An artesian well represents a natural plumbing system where groundwater is brought to the surface powered only by geology and physics. This type of well taps into an underground reservoir that is naturally pressurized. The water is forced upward through the wellbore, sometimes reaching the surface and flowing freely, eliminating the need for an electric pump or other machinery. This self-flowing phenomenon is entirely dependent on the unique structural arrangement of rock layers beneath the ground.
Defining the Artesian System
An artesian well draws water from a confined aquifer, a water-bearing layer trapped deep beneath the surface. This aquifer is composed of a permeable material, such as porous sandstone or fractured limestone, which holds and transmits water. The water within this permeable layer is sealed in place by two surrounding, impermeable layers, often made of clay or dense shale.
These impermeable layers, known as confining layers, prevent the water from escaping upward or downward, allowing pressure to build within the system. This geologic structure differentiates a confined aquifer from an unconfined aquifer, where the water table is open to the atmosphere. The system is completed by a recharge area, the location, often at a higher elevation, where the permeable rock layer is exposed to the surface.
Rainfall and surface water seep into the ground at this recharge area and travel underground, filling the confined aquifer. This distant, elevated point of entry provides the energy that drives the artesian system. The water is constantly being replenished, moving from a high-elevation source into a deep, pressurized storage area.
The Role of Hydrostatic Pressure
The water flows up the well without pumping due to hydrostatic pressure, which is the pressure exerted by a fluid at rest due to gravity. Since the water in the confined aquifer enters the system at a higher elevation in the distant recharge area, that higher column of water exerts a downward pressure on all the water trapped lower in the aquifer.
When a well is drilled into this confined aquifer, it provides a pathway for the pressurized water to escape. The water immediately begins to rise in the well casing, trying to reach the same height as the water source in the elevated recharge area. This theoretical elevation to which the water would rise is called the potentiometric surface, an imaginary plane that represents the pressure head of the water within the confined aquifer.
The flow occurs because the wellhead has been drilled to a point that is lower than this potentiometric surface. The pressure created by the difference in elevation between the recharge area and the well site pushes the water up the well. The pressure pushes the water up the well until the weight of the water column in the well pipe balances the pressure in the aquifer.
Flowing Versus Non-Flowing Wells
The term “artesian well” is applied to any well that taps a confined aquifer, meaning the water level rises above the top of the aquifer itself due to pressure. Not all artesian wells flow freely at the surface, which leads to a distinction between the two types.
A flowing artesian well is one where the potentiometric surface of the aquifer is located above the ground level at the well site. When the well is drilled, the water naturally rises and overflows onto the land surface without assistance.
A non-flowing artesian well is far more common and occurs when the potentiometric surface is above the top of the aquifer but remains below the ground level. The water rises part of the way up the well casing under its own pressure but does not reach the surface. This type of well still provides a benefit, as only minor pumping is required to bring the water the remaining distance to the surface.